The present invention relates to a switching semiconductor device and a switching circuit employed in a radio-frequency communication apparatus such as a cellular telephone.
Recently, a high-frequency switching circuit which switches over a high-frequency signal transmitted or received through an antenna between a transmitter-side circuit and a receiver-side circuit has been widely employed in a mobile communication apparatus represented by a cellular telephone. Important electric characteristics of the high-frequency switching circuit are an insertion loss characteristic in an ON state and an isolation characteristic in an OFF state.
A gallium arsenide (GaAs)-based field-effect transistor (FET) manufactured on a substrate consisting of GaAs has been conventionally used so that the high-frequency switching circuit can exhibit both the insertion loss characteristic and the isolation characteristic. Even in an era in which GaAs is replaced by silicon (Si) or silicon germanium (SiGe) as a material for a high-frequency component of the mobile communication apparatus, GaAs-based compound semiconductor tends to be still used for the switching IC device.
FIG. 17 depicts a circuit configuration of a high-frequency switching circuit composed of conventional FETs (FET1 and FET2) and resistors (R1 and R2) provided between gates and control terminals (CTL1 and CTL2) of the FET1 and FET2, respectively (see, for example, K. Miyatsuji and D. Ueda, “A GaAs High Power RF single Pole Dual Throw Switch IC for Digital Mobile Communication System”, IEEE Journal of Solid-state circuits, Vol. 30, No. 9, pp. 979–983, September 1995). If a voltage of 0 V is applied to the control terminal CTL1 of the FET1 and a voltage of −5 V is applied to the control terminal CTL2 of the FET2, the switching circuit turns into an ON state. Conversely, if a voltage of −5 V is applied to the control terminal CTL1 of the FET1 and a voltage of 0 V is applied to the control terminal CTL2 of the FET2, the switching circuit turns into an OFF state.
Device parameters, based on which the important electric characteristics, i.e., the insertion loss characteristic and the isolation characteristic of the switching circuit are determined, are an ON resistance Ron which is a resistance between a drain and a source when each FET is in the ON state and an OFF capacitance Coff which is a capacitance between the drain and the source when the FET is in the OFF state.
It is necessary to reduce the ON resistance Ron so as to reduce an insertion loss in the ON state. In addition, it is necessary to reduce the OFF capacitance Coff so as to improve the isolation characteristic in the OFF state.
However, there is a tradeoff relationship between the ON resistance Ron and the OFF capacitance Coff. In other words, if a distance between the drain and the source of the FET is shortened so as to reduce the ON resistance Ron, the OFF capacity Coff is contrarily increased, resulting in deterioration in the isolation characteristic.